The present invention relates to wireless telecommunications systems which use spread-spectrum methods.
One of the most important tools in telecommunications is spread spectrum methods. For example, in a direct-sequence spread-spectrum (xe2x80x9cDS-SSxe2x80x9d) transmission, the signal is spread by a spreading code which is known to both the transmitter and receiver. (The spreading code is merely a long pseudo-random bit sequence, that is, a sequence of bits which appears random but is determined by the input to a generator and is, therefore, reproducible. The sequence is generated identically, at both the transmitter and receiver, by custom hardware.) At the receiving end, digital filtering methods can be used to selectively recognize only the signals which are encoded with the expected pseudo-random bit sequence. Since the spreading code is used to separate signals which share the same spectrum space, these methods are also known as CDMA (code-division-multiple-access). Samples of the spread signal are called xe2x80x9cchips.xe2x80x9d The chip rate is usually much faster than the bit rate; the ratio is called the xe2x80x9cspreading factorxe2x80x9d or the xe2x80x9cprocessing gainxe2x80x9d.
The term xe2x80x9cspread spectrumxe2x80x9d is also used to refer to two other techniques: xe2x80x9cfrequency-hoppingxe2x80x9d systems, in which the transmitter frequency changes in some way which the receiver can predict; and xe2x80x9cchirpxe2x80x9d modulation or Pulse-FM in which a carrier is swept over a wide band during a given pulse interval.
CDMA methods are commonly used in cell phone systems. In such a system, adjacent base stations must have different spreading sequences (long pseudo-noise or xe2x80x9cPNxe2x80x9d codes), and the mobile unit must be able to lock onto the correct long code (spreading sequence) for each base station it may interface to. The mobile unit will already know the set of possible long codes which it may encounter, but will not know a priori which long code it will encounter when switched on. In most systems the mobile unit will also not know what the received long code offset is, that is, the timing of the transmission of the long code is not known. However, it is highly desirable for the mobile unit to acquire the received long code quickly. This is an essential step during the handoff process from one base station to another, as well as during initial acquisition when the mobile unit is powered on.
Thus the acquisition of the long code during initial acquisition or handoff is a critical bottleneck. A technique for accelerating this has been proposed in the NTT DoCoMo (xe2x80x9cNippon Telegraph and Telephone Mobile Communications Network, Inc.xe2x80x9d) System. In this system, each base station transmits a signal on a xe2x80x9cpilotxe2x80x9d (or xe2x80x9cperchxe2x80x9d) channel which helps the mobile system to acquire the correct long code for that base station.
The DoCoMo System introduced some features to make the PN acquisition of the long code simpler. Firstly, the DoCoMo system divides the entire set of spreading codes into code groups. Each base station or cell in the system transmits the identity of the code group corresponding to its spreading sequence. This is done in the following fashion.
Each cell intermittently broadcasts a common short code marker (256 chips long in the case of the DoCoMo system); this marker is referred to henceforth as xe2x80x9cSC0xe2x80x9d. The SC0 symbol is not encoded by the base station""s long code.
Thus, a mobile receiver can simply search for the particular short code (xe2x80x9cSC0xe2x80x9d) which is transmitted periodically by every base station. One of several Secondary Short Codes (SSC1, . . . , SSCN) is transmitted time aligned and overlapping the SC0 marker. When the mobile unit finds SC0, it can also look to see which of the short codes SCk (1 less than =k less than =N) is being broadcast synchronously with the SC0 code. The SCk code will show which group of transmitters the base station belongs to. The receiver then uses this information to shorten its search through the complete set of codes. Once this information has been acquired, there are still two ambiguities which must be resolved: the receiver must still identify which long code, within the reduced group of possible long codes, is being broadcast; and the receiver must still determine the phase of the long code.
That is, within the timing architecture of the DoCoMo system, a long-code-masked symbol is broadcast once in every ten symbols. Since there are 160 symbols in the complete long code, the long-code-masked symbols will be broadcast 16 times before the long code has repeated once. Thus in the DoCoMo system the receiver has to discover which of the 16 repetitions of the short code (within the long code period) has been detected. (Since it is not known which long code is being used, it is necessary to check all possible phases of the reduced set of possible long codes.) See Higuchi Et al., xe2x80x9cFast cell search algorithm in DS-CDMA mobile radio using long spreading codes,xe2x80x9d 1997 IEEE 47th Vehicular Technology Conference vol. 3 pp. 1430-4, which is hereby incorporated by reference.
The present application discloses an improved mobile communications architecture, in which each base station broadcasts not only data which has been spread by that station""s long code word, but also (intermittently) code identification. The code identification data is a block code which includes multiple symbols, so that multiple intermittent transmissions are required to complete the transmission of the code identification data. This transmission lets the mobile station shorten the search for the base station""s spreading code in two ways: the code identification data gives at least some information about the long code itself; and the phase of the block code gives at least some information about the timing of the long code word.
This advantageously results in a system where the amount of searching which a mobile unit must do to acquire the correct code for a new base station is greatly reduced. This results in the further advantage of faster acquisition of a new base station during hand off.